Reciprocating engines are widely used as a convenient means for converting chemical energy into motion. Several approaches have been proposed for diagnosing the state of health of such engines, and one of the most popular is by vibration analysis. Perhaps in its most basic form, an experienced maintenance technician or operator may often sense that something is wrong by the sound of an engine, aurally detecting undesirable vibrations. In certain circumstances, particularly with the more serious problems, he may even be able to isolate, or at least guess at the probable cause.
More sophisticated and much more sensitive techniques for monitoring the health of an engine involve the detecting of the vibrations generated in operation, usually by use of a suitable transducer, with signal analysis using suitable signal processing algorithms and processing hardware. The underlying concept is that every machine with moving parts will vibrate in response to the excitations employed on its components. Variations in the excitation forces, the machine's components or their interconnections will change the vibration pattern. Consequently periodic monitoring of the machine's vibration signature can provide useful information regarding its components and the excitations acting upon them.
A commonly used algorithm for analysing vibrations is the Fast Fourier Transformation (FFT). This numerical transformation enables the conversion of the vibration signals from the time domain to the frequency domain, and provides the magnitudes and phase angles of the harmonic terms comprising the vibration signals. It is known that a malfunction such as a misfiring cylinder will give rise to characteristic additional harmonic terms or will alter the relative intensities of existing terms, and, in principle, such disturbances can be used for diagnostic purposes. In practice, such diagnoses are not trivial however. Whereas FFT analysis of transducer output has been widely and successfully used for maintenance and monitoring purposes on rotating engines such as aero-engines [Eshleman 1995; Davies 1998; Kapadia N. S. et al. U.S. Pat. No. 4,488,240], it has proved much more difficult to identify faults in reciprocating-cylinder type engines, the vibration characteristics of which tend to be more complex.
Pioneering work in the diagnosis of reciprocating engine faults using vibration analysis includes the following. Nurhadi et al. [Nurhadi, I., Bagiasna, K. and Wediyanto. Signature Analysis of 4-Stroke 1-Cylinder Engine. SAE technical paper 932011, 1993 (Society of Automobile Engineers, Warrendale, Pa.)] were able to identify lubrication problems. Autar [Autar, R. K. An Automated Diagnostic Expert System for Diesel Engines. J. Engineering for Gas Turbines and Power, 1996, 118, pp. 673–679] describes a system that enables the detection of malfunctions such as cylinder compression and combustion faults, valve related faults and piston slap. Macian et al. [Macian, M., Lerma, M. J. and Barila, D. Condition Monitoring of Thermal Reciprocating Engines Through Analysis of Rolling Block Oscillations. SAE technical paper 980116, 1998 (Society of Automobile Engineers, Warrendale, Pa.)] also conclude that the vibration signature of the engine block can provide useful information regarding malfunctioning in one of the cylinders. Also, deBotton et al. [deBotton, G., Ben-Ari, J., Itzhaki, R. and Sher, E. Vibration Signature Analysis as a Fault Detection Method for SI Engines. SAE technical paper 980115, 1998 (Society of Automobile Engineers, Warrendale, Pa.). (also published in the 1998 SAE Transactions)] have demonstrated that the vibration signature of such engines can be used to identify the source of many malfunctions such as disconnected spark plug, narrow or wide spark plug gaps, early or late sparking, air leakage into the inlet manifold and loose engine support. Whereas all these problems result in additional vibrations that can be detected by a suitable transducer or other detection means, and resolved using FFT or similar algorithms, in real engines with localised malfunctioning, it has proved difficult to identify which cylinder is malfunctioning, and what type of fault is present. This is due to the complex nature of the reciprocating machines that incorporates a large number of moving parts. Using known methods results in the generation of complex waveforms which do not easily lend themselves to advanced universal root cause analyses schemes and thus require massive accompanying references data bases that need to be recorded for each individual type of engine.
In U.S. Pat. No. 5,633,456, a method is described for detecting cylinder misfiring and for identifying the corresponding cylinder. The method is based on sensing of the rotation of the crankshaft, comprising measurements of changes in the angular velocity of the crankshaft. Using a series of windows in the time domain the angular velocity is determined during the combustion in each of the cylinders in the engine. Then the velocity changes are determined by subtracting successive velocities. The method uses a specific filter to determine the occurrence of a misfire in a specific cylinder and a high-pass and low-pass filters to reduce the amount of noise associated with other sources of vibrations in the engine. Due to the complexity that arises from the necessity to analyse the waveform, the system must be carefully adjusted to each engine type.
In U.S. Pat. No. 3,783,681 a number of individual transducers are used to sense engine vibration, each transducer being connected to a different one of the cylinders that comprise the engine. The vibration data is used to assess the mechanical integrity of the cylinder, primarily surface and other erosion related faults that might lead to a failure, to increased fuel consumption etc. As all cylinders within an engine are situated within the cylinder block, in such a design, vibrations will be picked up from all cylinders, not just from the one ostensibly being monitored. This patent is directed to some technical issues and not on the concept of how to analyse and correlate the measured data.
In EP 245005, measurements of angular velocity oscillations of the crankshaft are carried out with an optical encoder—these oscillations are referred to as torsional vibrations. The waveform is transformed to the frequency domain where both amplitude and phase angle are determined at orders of the crankshaft revolution frequency. A method is applied to analyze the torsional vibrations at the first harmonic order (accounting for both amplitude and phase) in order to obtain the required correction in fuel supply for each cylinder. However, for a complete diagnosis of the engine and for the disordered cylinder to be uniquely identified this method requires a database for the specific engine tested to have been previously compiled.
None of the above references provide a universal systematic method which is applicable for large numbers of engine, and in any case generally rely on comparison of the results obtained with a pre-acquired reference data base.
There is therefore a need for, and it is thus an aim of the present invention to provide, a method and system for monitoring the health-condition of an engine that overcomes the limitations of previously known methods and systems, in particular, such a method and system that is more sensitive and informative than known methods and systems.
It is another aim of the present invention to provide such a method and system for detecting a state of imbalance existing between the various cylinders of an internal combustion engine.
It is another aim of the present invention to provide such a method and system to enable the identification of a cylinder of a reciprocating engine which has or is developing a fault.
It is another aim of the present invention to provide such a method and system that enables the distinguishing between cases in which the operation of a disordered cylinder, typically the combustion process therein, is stronger or weaker than in the other cylinders.
It is a further aim of the invention to provide such a method and system for analysing and identifying the type of malfunction in the disordered cylinder.
It is a further aim of the present invention to provide such a system or method that requires the use of vibration sensing means situated in one location only on the engine, or alternatively a plurality of sensing means situated at different locations on the engine.
It is a further aim of the present invention to provide such a system or method in which engine monitoring may be accomplished while the engine is running under normal velocities and loads.
It is a further aim of the present invention to provide such a system or method wherein no substantial interference with the regular operation of the engine is required.
It is a further aim of the present invention to provide such a system or method wherein such monitoring may be accomplished without contact with the moving parts of the engine.
It is a further aim of the present invention to provide such a system or method such as to enable on-line real-time control of engine parameters such as fuel consumption rate, ignition timing, fuel injection timing, so as to correct or avoid an imbalanced state of operation.
It is a further aim of the present invention to provide such a system or method that may be successfully applied optionally without the need for referencing a previously compiled reference database.
It is a further aim of the present invention to provide such a system or method that may be universally applied to virtually any piston machine, including reciprocating or rotary engines, and in particular to internal combustion reciprocating engines, including diesel or spark-induced engines.
It is a further aim of the present invention to provide such a system or method for root cause analysis of faults, relating certain terms of a Fourier series expansion of a vibration waveform with an imbalanced state of operation of a reciprocating, and in particular internal combustion reciprocating engine.
As will become clear from the description and examples disclosed herein, the present invention achieves these and other aims by providing a method and system for diagnosing the health-condition of engines, in particular internal combustion reciprocating engines, that correlates the harmonic terms of the Fourier series representation of the engine vibration with the mechanical state of the engine. In particular, the characteristics of the harmonic components of the Fourier representation are monitored at one or more predetermined frequencies according to at least one first predetermined criterion, and the characteristics of these harmonic components are analysed according to at least one second predetermined criterion to determine the operational state of the engine correlated to said second criterion.
In DE4123030, the signal spectrum of the tested engine is provided and separated according to complete engine cycles. This is determined by measuring the force on a piezo element as a function of time. The measured force vs. time plot is subdivided such as to enable the measured force during complete engine cycles and relating to individual cylinder signals to be studied, and this is compared with the results obtained in the same way at equal loading and speed conditions for an internal combustion engine that was found empirically to be good. On the basis of this comparison, typically using standard deviation methods, it can be determined, according to the patent, whether the test engine operates within tolerable limits or not. Thus, there is no transformation of the vibration data into the frequency domain, less so any disclosure or suggestion of the analysis of such data according to the present invention.
EP 632261 is directed to detection of an abnormal resonant frequency to indicate that one or more cylinders is operating at an improper power level. Vibration signals from each cylinder (measured by individual transducers for each cylinder) are used to perform FFT analysis, and an average resonant frequency for each cylinder is obtained over a few engine cycles. Shifts in the averaged resonant frequency or a zero resonant frequency for only one of the cylinders is used as an indication that such a cylinder is experiencing a low power condition, such as misfiring. Thus, particular peak values of amplitudes are used to provide sought-after frequencies, and thus this patent neither discloses nor suggests the present invention.
EP 1050863 is concerned with tachometers for use in analysis work on engines, and in particular for providing rpm from vibration signatures of the engine. The vibration signature of an engine is determined as a power spectral density (PSD) plot, and the first three frequencies at which peak amplitudes occur are determined. These frequencies are theoretically multiples of the dominant frequency f, which is mathematically related to engine rpm. Thus, by knowing the type of engine, and thus the relationship between the multiples of f, and by determining f from the PSD plot, the engine rpm is determined. Therefore in this patent, what are actually being monitored are harmonics having characteristics according to very specific conditions, i.e., the harmonics having the three highest amplitude peaks, and then the frequencies at which this happens are provided. In this patent there is no actual monitoring at all at predetermined frequencies, since it is precisely the values of frequencies that are being sought in order to calculate the rpm, and are therefore not “predetermined”. In other words, a very specific set harmonics is being monitored in order to provide the sought-after frequencies. In contrast, in the present invention, a criterion is applied to the monitoring frequencies in order to find particular harmonics.
Also according to the patent, the location and identification of additional peaks enables identification of faults. Essentially, the frequencies at which the highest peaks are obtained are analysed, and the relative values of these frequencies are compared to the relative values that should be obtained for the type of engine (i.e., the theoretical multiples of f) to see if there is a malfunction. In other words, the frequencies obtained at particular harmonics (the harmonics having highest magnitudes) are analysed, in contrast to the present invention in which it is the characteristics of harmonics that are analysed at particular frequencies, and even at a single frequency. Furthermore, in this patent this analysis cannot be conducted at just one frequency, since an isolated peak without reference to the other two peaks is meaningless for providing multiples of f.